The present invention relates in general to multifocal lenses, and, more particularly, to a method of manufacturing a multifocal lens.
Known multifocal lenses are subject to many drawbacks, such as glare, reflection off internal surfaces, small field of vision, and the like.
A process for forming known multifocal lenses includes removing a piece of a single lens and replacing that removed piece with another lens portion. Such methods are disclosed in U.S. Pat. Nos. 3,915,723, 1,996,442, 1,373,633, 3,649,236, 3,563,057, 2,963,823 and 2,254,440. The replaced lens portion is formed of a material different from that of a single lens. This type of lens has many drawbacks, the principal ones being internal reflection and glare. Furthermore, the field of vision through the inserted piece is reduced from that field of vision otherwise available.
In another form of bifocal lens, i.e., the "Executive" type, the near vision section is ground on the same piece of glass as the distance vision part. Disadvantages of this type of lens are:
1. The difficulty of making a sharp step in the line of separation between two lens sections further creates the following disadvantages:
a. highly specialized equipment is required to manufacture such lenses; PA1 b. the manufacturing cost is thus relatively high; and PA1 c. step between two sections is not sharp.
2. When a near vision lens portion is ground on the bottom half of the distance vision lens portion, the top of the near vision portion will be a little below the top surface of the distance vision portion at the center, and since the near vision lens portion has the greater degree of curvature, that lens portion will be considerably below the far vision surface at the outer edges of the bifocal lens. In order to leave these edges (on the near vision section) at a practical thickness, the entire lens must be made relatively thick and heavy.
Much precise grinding and polishing is required to finish known multifocal lenses, and accordingly, the manufacturing process is quite difficult and expensive.